to be fair 5 million gold currently is 20usd=200k so that is 100$ for 1 mil, meaning 500$ mount...
However with BFA we are sure to see some increase in gold inflation so this is sure to change, but who knows by how much.
WoD it was 30k, it went to 80k for a good portion of legion and only raised after that once wow tokens were able to be used for any currency.